Ceramic heater, sensor element, and gas sensor

ABSTRACT

A heater section includes a plate-like ceramic body (a first substrate layer  1 , a second substrate layer  2 , and a third substrate layer  3 ) having a longitudinal direction (front-rear direction) and a short-length direction (left-right direction), and a heater  72  disposed within the plate-like ceramic body and including a lead section  79  and a heating section  76  connected to the lead section  79 . The heating section  76  includes a straight portion  78  extending along the longitudinal direction, and a lead side curved portion  77   b  connected to one of the ends of the straight portion  78  closer to the lead section  79 . The lead side curved portion  77   b  has a lower resistance per unit length than the straight portion  78  at one or more temperatures in the range of 700° C. to 900° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.15/238,855, filed on Aug. 17, 2016, which claims priority under 35U.S.C. § 119(a) to Japanese Patent Application No. 2015-164211, filed inJapan on Aug. 21, 2015. The entire contents of U.S. patent applicationSer. No. 15/238,855 and the Japanese Patent Application No. 2015-164211are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a ceramic heater, a sensor element, anda gas sensor.

2. Description of the Related Art

There has been known ceramic heaters including a ceramic sheet and aheater pattern that is folded a plurality of times in the longitudinaldirection of the ceramic sheet (for example, PTL 1). The heater patterndisclosed in PTL 1 includes straight conducting segments extending alongthe longitudinal direction, a curved conducting segment connecting thestraight conducting segments, and a pair of electricity conductingpatterns.

CITATION LIST Patent Literature

-   PTL 1: Japanese Patent No. 4826461

SUMMARY OF THE INVENTION

Some of the ceramic heaters contain an alkali metal or an alkaline-earthmetal. When the heating element heats, such a metal is likely to becationized and thus to be attracted to the low-potential side. Also,oxygen or any other element bound to the alkali metal or alkaline-earthmetal may be ionized into anion and thus to be attracted to thehigh-potential side. These phenomena are called migration. In a case,ions moved by migration react with the heating element, therebydegrading the heating element in such a manner that a line of thepattern is thinned or broken.

The present invention is intended to solve this problem, and a majorobject of the invention is to prevent degradation of the heating elementresulting from migration.

In order to achieve the major object, the following is provided.

A ceramic heater of the present invention includes:

a plate-like ceramic body having a longitudinal direction and ashort-length direction; and

a heating element disposed within the plate-like ceramic body, theheating element including a lead section and a heating section connectedto the lead section,

wherein the heating section includes a straight portion whose lengthdirection is along the longitudinal direction and at least one lead sidecurved portion connected to one of the ends of the straight portioncloser to the lead section, and wherein the closest to the lead sectionof the at least one lead side curved portion has a lower resistance perunit length than the straight portion at one or more temperatures in therange of 700° C. to 900° C.

In the first ceramic heater, the heating section includes the straightportion and the lead side curved portion. The closest to the leadsection of the at least one lead side curved portion has a lowerresistance per unit length than the straight portion at one or moretemperatures in the range of 700° C. to 900° C. Thus, the closest leadside curved portion to the lead section has a lower heating density(quantity of heat generation per unit length) than the straight portionat one or more temperatures in the range of 700° C. to 900° C., andaccordingly, the temperature increase of this curved portion issuppressed. In general, since the closest lead side curved portion isclose to the lead section, the region between the closest lead sidecurved portion and the lead section tends to have a large potentialgradient, and thus migration is likely to occur. In addition, alkalimetals, alkaline-earth metals, and the like are more likely to ionize astemperature increases. In the first ceramic heater of the presentinvention, ionization is prevented by suppressing the temperatureincrease of the closest lead side curved portion to the lead section, inwhich migration is liable to occur. Thus, at least one of the closestlead side curved portion and the lead section can be prevented frombeing degraded by migration. Consequently, the degradation of theheating element is reduced.

A second ceramic heater of the present invention includes:

a plate-like ceramic body having a longitudinal direction and ashort-length direction; and

a heating element disposed within the plate-like ceramic body, theheating element including a lead section having a positive lead and aheating section connected to the lead section.

The heating section includes a straight portion that are arranged alongthe longitudinal direction and whose length direction is along thelongitudinal direction of the plate-like ceramic body, and at least onelead side curved portion connected to one of the ends of the straightportion closer to the lead section. The closest to the positive lead ofthe at least one lead side curved portion has a lower resistance perunit length than the straight portion at one or more temperatures in therange of 700° C. to 900° C.

In the second ceramic heater, the heating section includes a straightportion and at least one lead side curved portion. The closest to thepositive lead of the at least one lead side curved portion has a lowerresistance per unit length than the straight portion at one or moretemperatures in the range of 700° C. to 900° C. Thus, the closest leadside curved portion to the positive lead has a lower heating densitythan the straight portion at one or more temperatures in the range of700° C. to 900° C., and accordingly, the temperature increase of thiscurved portion is suppressed. In general, since the closest lead sidecurved portion is close to the positive lead, the region between theclosest lead side curved portion and the positive lead tends to have alarge potential gradient, and thus migration is likely to occur. Inaddition, alkali metals, alkaline-earth metals, and the like are morelikely to ionize as temperature increases. In the second ceramic heaterof the present invention, ionization is prevented by suppressing thetemperature increase of the closest lead side curved portion to thepositive lead, in which migration is liable to occur. Thus, at least oneof the closest lead side curved portion and the lead section can beprevented from being degraded by migration. Consequently, thedegradation of the heating element is reduced.

In the first and the second ceramic heater, the ratio R1/R2 of unitresistance R1 [μΩ/mm] being the resistance per unit length of theclosest lead side curved portion to unit resistance R2 [Ω/mm] being theresistance per unit length of the straight portion may be 0.87 or lessat one or more temperatures in the above temperature range. Thus, thedegradation of the heating element resulting from migration can befurther reduced. Preferably, the unit resistance ratio R1/R2 is 0.80 orless at one or more temperatures in the above temperature range.

In the first and the second ceramic heater, the cross section of theclosest lead side curved portion, taken in the direction perpendicularto the length direction thereof may have a larger area than that of thestraight portion. Thus, the resistance per unit length of the closestlead side curved portion tends to be lower than that of the straightportion. In this instance, preferably, the ratio S2/S1 of the area S2[mm²] of the cross section of the straight portion taken in thedirection perpendicular to the length direction thereof to the area S1[mm²] of the cross section of the closest lead side curved portion takenin the direction perpendicular to the length direction thereof is 0.87or less. Thus, the unit resistance ratio R1/R2 tends to be 0.87 or lessat one or more temperatures in the above temperature range. Morepreferably, the cross-sectional area ratio S2/S1 is 0.80 or less.

In the first and the second ceramic heater, the closest lead side curvedportion may have a lower volume resistivity than the straight portionsat one or more temperatures in the above temperature range. Thus, theresistance per unit length of the closest lead side curved portion tendsto be lower than that of the straight portion. In this instance,preferably, the ratio ρ1/ρ2 of the volume resistivity ρ1 [μΩ·cm] of theclosest lead side curved portion to the volume resistivity ρ2 [μΩ·cm] ofthe straight portion is 0.87 or less at one or more temperatures in theabove temperature range. Thus, the unit resistance ratio R1/R2 tends tobe 0.87 or less at one or more temperatures in the above temperaturerange. More preferably, the volume resistivity ratio ρ1/ρ2 is 0.80 orless at one or more temperatures in the above temperature range.

In the first and the second ceramic heater, the heating section may havefour or more straight portions arranged along the short-length directionof the plate-like ceramic body. In this structure, the at least one leadside curved portion connects a pair of the straight portions adjacent toeach other in the short-length direction at one of the ends of eachadjacent straight portion closer to the lead section, and the heatingsection includes a plurality of non-lead side curved portions, eachconnecting a pair of the straight portions adjacent to each other in theshort-length direction at one of the ends of each adjacent straightportion far from the lead section.

A sensor element according to the present invention includes

the first or the second ceramic heater according to any of theabove-described embodiments.

The sensor element is used to detect the concentration of a specific gasin a measurement-object gas.

Since the sensor element includes the ceramic heater of any of theabove-described embodiments, the sensor element can produce the sameeffect as the first and the second ceramic heater, for example, theeffect of reducing the degradation of the heating element caused bymigration.

A gas sensor of the present invention includes the above-describedsensor element.

Since the sensor includes the sensor element including the ceramicheater of any of the above-described embodiments, the sensor can producethe same effect as the ceramic heater and the sensor element of thepresent invention, for example, the effect of reducing the degradationof the heating element caused by migration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an example of the structure of agas sensor 100.

FIG. 2 is a sectional view taken along line A-A in FIG. 1 .

FIG. 3 is an explanatory view of a heater 72A according to amodification.

FIG. 4 is an explanatory view of a heater 72B according to amodification.

FIG. 5 is an explanatory view of a heater 72C according to amodification.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described withreference to the drawings. FIG. 1 is a schematic sectional view of anexample of the structure of a gas sensor 100 of an embodiment of thepresent invention. FIG. 2 is a sectional view taken along line A-A inFIG. 1 . The gas sensor 100 is such that a sensor element 101 thereindetects the concentration of a specific gas, such as NOx gas, in ameasurement-object gas, such as exhaust gas from automobiles. The sensorelement 101 is in the shape of a long rectangular parallelepiped. Thelongitudinal direction (lateral direction in FIG. 1 ) of this sensorelement 101 is defined as the front-rear direction, and the thicknessdirection (vertical direction in FIG. 1 ) of the sensor element 101 isdefined as the vertical direction. The width direction (perpendicular tothe front-rear direction and the vertical direction) of the sensorelement 101 is defined as the left-right direction.

The sensor element 101 has a multilayer structure defined by a stack ofsix oxygen ion conductive solid electrolyte layers made of, for example,zirconia (ZrO₂) that are a first substrate layer 1, a second substrate2, a third substrate layer 3, a first solid electrolyte layer 4, aspacer layer 5, and a second solid electrolyte layer 6 stacked in thatorder from the lower side of the figure. The solid electrolyte of thesix layers is densely airtight. The sensor element 101 is manufacturedby stacking, for example, ceramic green sheets that have been processedaccording to the respective layers and provided with the respectivecircuit patterns by printing, and sintering the stack into one body.

In the sensor element 101, at one end thereof, a gas inlet 10, a firstdiffusion controlling portion 11, a buffering space 12, a seconddiffusion controlling portion 13, a first internal void space 20, athird diffusion controlling portion 30, and a second internal void space40 are formed so as to communicate one after another in that orderbetween the lower surface of the second solid electrolyte layer 6 andthe upper surface of the first solid electrolyte layer 4.

The gas inlet 10, the buffering space 12, the first internal void space20, and the second internal void space 40 are each a space in the sensorelement 101 formed by boring the spacer 5 in such a manner that theupper side is defined by the lower surface of the second solidelectrolyte layer 6, the lower side is defined by the upper surface ofthe first solid electrolyte layer 4, and the side walls are defined bythe side surfaces of the spacer 5.

The first diffusion controlling portion 11, the second diffusioncontrolling portion 13, and the third diffusion controlling portion 30are each provided with two laterally long slits therein (whose openinghas a longitudinal direction perpendicular to the figure). The portionfrom the gas inlet 10 to the second internal void space 40 may bereferred to as a gas delivering section.

At a position more distant from the end side than the gas deliveringsection, a reference gas introducing space 43 is formed between theupper surface of the third substrate 3 and the lower surface of thespacer 5 in such a manner that the side wall is defined by a sidesurface of the first solid electrolyte layer 4. Into the reference gasintroducing space 43, for example, air is introduced as a reference gasfor NOx concentration measurement.

An air introducing layer 48 is a porous layer made of a porous ceramic,and to which a reference gas is introduced through the reference gasintroducing space 43. Also, the air introducing layer 48 is disposed soas to cover a reference electrode 42.

The reference electrode 42 is disposed between the upper surface of thethird substrate 3 and the first solid electrolyte layer 4, and isprovided therearound with the air introducing layer 48 communicatingwith the reference gas introducing space 43. The reference electrode 42is used for measuring the oxygen concentration (oxygen partial pressure)in the first internal void space 20 and the second internal void space40, as will be described later.

The gas inlet 10 in the gas delivering section is open to an externalspace, and through which a measurement-object gas is introduced into thesensor element 101 from the external space. The first diffusioncontrolling portion 11 gives a predetermined diffusion resistance to themeasurement-object gas that has been introduced through the gas inlet10. The buffering space 12 is intended to deliver the measurement-objectgas introduced through the first diffusion controlling portion 11 to thesecond diffusion controlling portion 13. The second diffusioncontrolling portion 13 gives a predetermined diffusion resistance to themeasurement-object gas that is being introduced to the first internalvoid space 20 through the buffering space 12. When a measurement-objectgas is delivered to the first internal void space 20 from the outside ofthe sensor element 101, the measurement-object gas rapidly taken intothe sensor element 101 through the gas inlet 10 by pressure fluctuationof the measurement-object gas (for the case of measuring automotiveexhaust gas, pulsation of exhaust gas pressure) in the external space ispassed through the first diffusion controlling portion 11, the bufferingspace 12, and the second diffusion controlling portion 13 to cancel thefluctuation in concentration of the measurement-object gas before beingdelivered to the first internal void space 20, instead of being directlydelivered to the first internal void space 20, and is then delivered tothe first internal void space 20. Thus, the fluctuation in concentrationof the measurement-object gas delivered to the first internal void space20 is substantially negligible. The first internal void space 20 isintended to control the oxygen partial pressure of themeasurement-object gas delivered through the second diffusioncontrolling portion 13. The oxygen partial pressure is controlled by theoperation of a main pump cell 21.

The main pump cell 21 is an electrochemical pump cell including an innerpump electrode 22 having a ceiling electrode portion 22 a disposed oversubstantially the entire lower surface of the portion of the secondsolid electrolyte layer 6 opposing the first internal void space 20; anouter pump electrode 23 disposed so as to be exposed to the externalspace in the region of the upper surface of the second solid electrolytelayer 6 corresponding to the ceiling electrode portion 22 a; and thesecond solid electrolyte layer 6 between the inner and the outer pumpelectrode.

The inner pump electrode 22 is formed so as to be across the upper andthe lower solid electrolyte layer (the second solid electrolyte layer 6and the first solid electrolyte layer 4) defining the first internalvoid space 20, and the spacer layer 5 defining the side walls. Morespecifically, the ceiling electrode portion 22 a is formed on the lowersurface of the second solid electrolyte layer 6 defining the ceiling ofthe first internal void space 20; a bottom electrode portion 22 b isformed on the upper surface of the first solid electrolyte layer 4defining the bottom of the first internal void space 20; and sideelectrode portions (not shown) are formed on the side wall surfaces(inner surfaces) of the spacer layer 5 defining the side walls of thefirst internal void space 20 so as to connect the ceiling electrodeportion 22 a and the bottom electrode portion 22 b. Thus, the inner pumpelectrode 22 is in the form of a tunnel.

The inner pump electrode 22 and the outer pump electrode 23 are porouscermet electrodes (for example, cermet electrodes of Pt containing 1% ofAu and ZrO₂). The inner pump electrode 22, which comes in contact withthe measurement-object gas, is made of a material whose ability toreduce the NOx in the measurement-object gas has been weakened.

The main pump cell 21 is configured so that the oxygen in the firstinternal void space 20 can be pumped out to the external space orexternal oxygen can be pumped into the first internal void space 20 byapplying a desired pump voltage Vp0 between the inner pump electrode 22and the outer pump electrode 23 to pass a positive or negative pumpcurrent Ip0 between the inner pump electrode 22 and the outer pumpelectrode 23.

In addition, in order to detect the oxygen concentration (oxygen partialpressure) in the atmosphere in the first internal void space 20, theinner pump electrode 22, the second solid electrolyte layer 6, thespacer layer 5, the first solid electrolyte layer 4, the third substratelayer 3, and the reference electrode 42 constitute an electrochemicalsensor cell, that is, a main pump controlling oxygen partial pressuredetecting sensor cell 80.

By measuring the electromotive force V0 of the main pump controllingoxygen partial pressure detecting sensor cell 80, the oxygenconcentration (oxygen partial pressure) in the first internal void space20 can be known. Furthermore, the pump current Ip0 is controlled byfeedback-controlling the pump voltage Vp0 of a variable power supply 24so that the electromotive force V0 can be constant. Thus, the oxygenconcentration in the first internal void space 20 is kept constant at aspecific value.

The third diffusion controlling portion 30 gives a predetermineddiffusion resistance to the measurement-object gas in which the oxygenconcentration (oxygen partial pressure) has been controlled in the firstinternal void space 20 by the operation of the main pump cell 21 anddelivers the measurement-object gas to the second internal void space40.

The second internal void space 40 is formed as a space in which aprocess for measuring the nitrogen oxide (NOx) concentration in themeasurement-object gas delivered through the third diffusion controllingportion 30 is performed. NOx concentration is measured mainly in thesecond internal void space 40 in which the oxygen concentration iscontrolled with an auxiliary pump cell 50, and, in addition, by theoperation of a measurement pump cell 41.

The second internal void space 40 further controls, with the auxiliarypump cell 50, the oxygen partial pressure in the measurement-object gasthat has been subjected to control of the oxygen concentration (oxygenpartial pressure) in advance in the first internal void space 20 andthen delivered through the third diffusion controlling portion 30. Thus,the oxygen concentration in the second internal void space 40 isprecisely kept constant, and accordingly, the gas sensor 100 is allowedto highly precisely measure NOx concentration.

The auxiliary pump cell 50 is an auxiliary electrochemical pump cellincluding an auxiliary pump electrode 51 having a ceiling electrodeportion 51 a disposed over substantially the entire lower surface of thesecond solid electrolyte layer 6 that feces the second internal voidspace 40; the outer pump electrode 23 (an appropriate electrode outsidethe sensor element 101 suffices without being limited to the outer pumpelectrode 23); and the second solid electrolyte layer 6.

The auxiliary pump cell 51 is disposed in the second internal void space40 at a tunnel shaped structure, in the same manner as the inner pumpelectrode 22 disposed in the first internal void space 20. Morespecifically, the ceiling electrode portion 51 a is formed on theportion of the second solid electrolyte layer 6 defining the ceiling ofthe second internal void space 40; a bottom electrode portion 51 b isformed on the portion of the first solid electrolyte layer 4 definingthe bottom of the second internal void space 40; and side electrodeportion (not shown) connecting the ceiling electrode portion 51 a andthe bottom electrode portion 51 b are formed on the side wall surfacesof the spacer layer 5 defining the side walls of the second internalvoid space 20. As with the inner pump electrode 22, the auxiliary pumpelectrode 51 is made of a material whose ability to reduce the NOx inthe measurement-object gas has been weakened.

The auxiliary pump cell 50 is configured so that the oxygen in theatmosphere in the second internal void space 40 can be pumped out to theexternal space or external oxygen can be pumped into the second internalvoid space 40 by applying a desired pump voltage Vp1 between theauxiliary pump electrode 51 and the outer pump electrode 23.

In addition, in order to control the oxygen partial pressure in theatmosphere in the second internal void space 40, the auxiliary pumpelectrode 51, the reference electrode 42, the second solid electrolytelayer 6, the spacer layer 5, the first solid electrolyte layer 4, andthe third substrate layer 3 constitute an electrochemical sensor cell,that is, an auxiliary pump controlling oxygen partial pressure detectingsensor cell 81.

The pumping of the auxiliary pump cell 50 is operated by a variablepower supply 52 whose voltage is controlled according to theelectromotive force V1 detected by the auxiliary pump controlling oxygenpartial pressure detecting sensor cell 81. Thus, the oxygen partialpressure in the atmosphere in the second internal void space 40 iscontrolled to be a low partial pressure that does not substantiallyaffect NOx measurement.

In addition, pump current Ip1 is used for controlling the electromotiveforce of the main pump controlling oxygen partial pressure detectingsensor cell 80. More specifically, a pump current Ip1 is input as acontrol signal to the main pump controlling oxygen partial pressuredetecting sensor cell 80 to control the electromotive force V0 of thesensor cell 80, thereby controlling the gradient of the oxygen partialpressure in the measurement-object gas that is being delivered to thesecond internal void space 40 through the third diffusion controllingportion 30 to be always constant. In use as a NOx sensor, the oxygenconcentration in the second internal void space 40 is about 0.001 ppmand is thus kept constant by the operation of the main pump cell 21 andthe auxiliary pump cell 50.

The measurement pump cell 41 measures the NOx concentration in themeasurement-object gas in the second internal void space 40. Themeasurement pump cell 41 is an electrochemical pump cell including ameasurement electrode 44 disposed on the upper surface of the firstsolid electrolyte layer 4 that faces the second internal void space 40,apart from the third diffusion controlling portion 30; the outer pumpelectrode 23; the second solid electrolyte layer 6; the spacer electrode5; and the first solid electrolyte layer 4.

The measurement electrode 44 is a porous cermet electrode. Themeasurement electrode 44 functions also as a NOx reducing catalyst thatreduces the NOx in the atmosphere in the second internal void space 40.The measurement electrode 44 is covered with a fourth diffusioncontrolling portion 45.

The fourth diffusion controlling portion 45 is a film made of a porousceramic. The fourth diffusion controlling portion 45 not only functionsto restrict the amount of NOx flowing into the measurement electrode 44,but also functions as a protective film of the measurement electrode 44.The measurement pump cell 41 pumps out oxygen produced by decompositionof the nitrogen oxides in the atmosphere around the measurementelectrode 44 and detects the amount of the oxygen as pump current Ip2.

In addition, in order to detect the oxygen partial pressure in theatmosphere around the measurement electrode 44, the first solidelectrolyte layer 4, the third substrate layer 3, the measurementelectrode 44, and the reference electrode 42 constitute anelectrochemical sensor cell, that is, a measurement pump controllingoxygen partial pressure detecting sensor cell 82. A variable powersupply 46 is controlled according to the electromotive force V2 detectedby the measurement pump controlling oxygen partial pressure detectingsensor cell 82.

The measurement-object gas delivered to the second internal void space40 will reach the measurement electrode 44 through the fourth diffusioncontrolling portion 45 with an oxygen partial pressure controlled. Thenitrogen oxide in the measurement-object gas around the measurementelectrode 44 is reduced (2NO→N₂+O₂) to produce oxygen. The oxygen thusproduced will be pumped by the measurement pump cell 41. At this time,the voltage Vp2 of the variable power supply 46 is controlled so thatthe electromotive force V2 detected by the measurement pump controllingoxygen partial pressure detecting sensor cell 82 can be constant. Sincethe amount of oxygen produced around the measurement electrode 44 isproportional to the nitrogen oxide concentration in themeasurement-object gas, the nitrogen oxide concentration in themeasurement-object gas is calculated using the pump current Ip2 of themeasurement pump cell 41.

Alternatively, by combining the measurement electrode 44, the firstsolid electrolyte layer 4, the third substrate layer 3, and thereference electrode 42 to constitute an oxygen partial pressuredetecting mechanism as an electromechanical sensor cell, theelectromotive force generated according to the difference between theamount of oxygen produced by reduction of the NOx in the atmospherearound the measurement electrode 44 and the amount of oxygen containedin the reference air can be detected, and thus the NOx concentration inthe measurement-object gas can be determined from this electromotiveforce.

The electrochemical sensor cell 83 is made up of the second solidelectrolyte layer 6, the spacer layer 5, the first solid electrolytelayer 4, the third substrate layer 3, the outer pump electrode 23, andthe reference electrode 42. The oxygen partial pressure in themeasurement-object gas outside the sensor can be determined from theelectromotive force Vref generated in this sensor cell 83.

In the gas sensor 100 having the above-described structure, themeasurement pump cell 41 receives a measurement-object gas in which theoxygen partial pressure is kept low and constant (to the extent that NOxmeasurement is substantially not affected) by the operation of the mainpump cell 21 and the auxiliary pump cell 50. Thus, the NOx concentrationin the measurement-object gas can be known according to the pump currentIp2 caused by the operation of the measurement pump cell 41 to pump outoxygen produced by reduction of NOx substantially in proportion to theNOx concentration in the measurement-object gas.

Furthermore, the sensor element 101 includes a heater section 70 adaptedto heat the sensor element 101 and keep the sensor element 101 warm soas to increase the oxygen ion conductivity of the solid electrolyte. Theheater section 70 includes a heater connector electrode 71, a heater 72,a portion defining a through-hole 73, a heater insulating layer 74, anda portion defining a pressure release hole 75. The heater section 70also includes the first substrate layer 1, the second substrate layer 2,and the third substrate layer 3, each made of a ceramic. The heatersection 70 is structured as a ceramic heater including the heater 72,and the second and the third substrate layer 2 and 3 surrounding theheater 72. The heater 72 includes a heating section 76 and a leadsection 79, as shown in FIG. 2 .

The heater connector electrode 71 is in contact with the lower surfaceof the first substrate layer 1. By connecting the heater connectorelectrode 71 to an external power source, electricity is supplied fromthe outside to the heater section 70.

The heating section 76 of the heater 72 is an electric resistor disposedbetween the second substrate layer 2 and the third substrate layer 3 incontact therewith. The lead section 79 of the heater 72 is connected tothe heater connector electrode 71 through the through-hole 73. Whenelectricity is supplied through the heater connector electrode 71 fromthe outside, the heating section 76 heats to heat the solid electrolyteof the sensor element 101 and keep the solid electrolyte warm.

The heating section 76 of the heater 72 is embedded under the entireregion from the first internal void space 20 to the second internal voidspace 40 so as to control the entirety of the sensor element 101 to atemperature at which the solid electrolyte is activated.

The heater insulating layer 74 is an insulating layer formed of aninsulating material such as alumina over the upper and the lower surfaceof the heater 72. The heater insulating layer 74 is formed to provideelectrical insulation between the second substrate layer 2 and theheater 72 and electrical insulation between the third substrate layer 3and the heater 72.

The pressure release hole 75 passes through the third substrate layer 3and communicates with the reference gas introducing space 43 and isintended to alleviate internal pressure increase with increasingtemperature in the heater insulating layer 74.

The heating section 76 and the lead section 79 of the heater 72 will nowbe described in detail. The heating section 76 is a resistance heatingelement and has a shape of a one-stroke strip connected to the leadsection 79, as shown in FIG. 2 . The heating section 76 includes aplurality (three, in the present embodiment) of curved portions 77 and aplurality (four, in the present embodiment) of straight portions 78. Thecurved portions 77 and the straight portions 78 are electricallyconnected in series. The heating section 76 is bilaterally symmetrical.

The straight portions 78 are arranged at regular intervals in theshort-length direction (left-right direction) of the sensor element 101.Each of the straight portions 78 extends along the longitudinaldirection (front-rear direction) of the sensor element 101. In thepresent embodiment, the straight portions 78 are arranged in such amanner that the length direction thereof is parallel to the front-reardirection. The rear end of the leftmost one of the straight portions 78is connected to a first lead 79 a that is a positive lead. The rear endof the rightmost one of the straight portions 78 is connected to asecond lead 79 b that is a negative lead.

Each curved portion 77 connects a pair of the straight portions 78adjacent to each other in the left-right direction. The curved portions77 include non-lead side curved portions 77 a, each connecting the frontends (far from the lead section 79) of a pair of the adjacent straightportions 78, and at least one lead side curved portion 77 b connectingthe rear ends (closer to the lead section 79) of a pair of the adjacentstraight portions 78. In the present embodiment, the curved portions 77include two non-lead side curved portions 77 a and one lead side curvedportion 77 b. Since the number of lead side curved portions 77 b is one,the lead side curved portion 77 b is the closest to the lead section 79of the at least one lead side curved portion and is, in addition, theclosest lead side curved portion to the first lead 79 a (positive lead).The lead side curved portion 77 b and the first lead 79 a are closestbetween points P1 and P2 in FIG. 2 . The lead side curved portion 77 band the second lead 79 b are also closest between points P3 and P4. Thedistance between points P1 and P2 will be referred to as distance L1,and the distance between points P3 and P4 will be referred to asdistance L2. Distance L1 is equal to distance L2. Each of the pluralityof curved portions 77 is bent to form a curve and in the shape of thearc of a semicircle. The curved portions 77 may have a linearly bentshape.

In the present embodiment, the heating section 76 is made of a cermetcontaining a noble metal and a ceramic (for example, platinum (Pt) andalumina (Al₂O₃)). The material of the heating section 76 may be anelectroconductive material such as a noble metal without being limitedto cermet. Examples of the noble metal used in the heating section 76include at least one of platinum, rhodium (Rh), gold (Au), and palladium(Pd), or an alloy of these noble metals.

The lead side curved portion 77 b that is the closest lead side curvedportion to the lead section and to the positive lead has a lowerresistance per unit length than the straight portions 78 at one or moretemperatures in the range of 700° C. to 900° C. to which the heatingsection 76 can be heated. In other words, the ratio R1/R2 of unitresistance R1 [μΩ/mm] that is the resistance per unit length of the leadside curved portion 77 b to unit resistance R2 [μΩ/mm] that is theresistance per unit length of the straight portions 78 is less than 1 atone or more temperatures in that temperature range. Thus, the lead sidecurved portion 77 b has a lower heating density (quantity of heatgeneration per unit length) than the straight portions 78 at one or moretemperatures in the range of 700° C. to 900° C., and accordingly, thetemperature increase of this curved portion is suppressed. Bycontrolling the unit resistance ratio R1/R2 to less than 1, thetemperature increase of the lead side curved portion 77 b can besuppressed, and the degradation of the heater 72 resulting frommigration (that will be described in detail later) can be reducedcompared to the case where the unit resistance ratio R1/R2 is 1 or more.

The length directions of the lead side curved portion 77 b and thestraight portions 78 correspond to the axis directions of the lead sidecurved portion 77 b and the straight portions 78, respectively; hence,current flows along these directions. Unit resistance R1 is defined bythe average of resistances per unit length of the lead side curvedportion 77 b. Similarly, unit resistance R2 is defined by the average ofresistances per unit length of the plurality of straight portions 78.Therefore, even if a portion of the lead side curved portion 77 b has ahigher resistance per unit length than the straight portions 78, theresistance per unit length of the lead side curved portion 77 b as awhole is lower than that of the straight portions 78. It is howeverpreferable that at least the closest portion (at points P1 and P3) ofthe lead side curved portion 77 b to the lead section 79 have a lowerresistance per unit length than unit resistance R2. More preferably, theresistance per unit length of any portion of the lead side curvedportion 77 b is lower than unit resistance R2. In the heating section76, preferably, the unit resistance ratio R1/R2 is less than 1 at anytemperature in the above temperature range. In the heating section 76,more preferably, the unit resistance ratio R1/R2 is 0.87 or less, morepreferably 0.80 or less, at one or more temperatures in the abovetemperature range. The unit resistance ratio R1/R2 may be 0.5 or more atany temperature in the above temperature range.

In the present embodiment, the lead side curved portion 77 b and thestraight portions 78 are made of the same material (cermet containingplatinum, as described above), and the area S1 [mm²] of the crosssection of the lead side curved portion 77 b taken in the directionperpendicular to the length direction thereof is larger than the area S2[mm²] of the cross section of the straight portions 78 taken in thedirection perpendicular to the length direction thereof. Hence, theheating section 76 has a cross-sectional area ratio S2/S1 of lessthan 1. In this instance, the unit resistance ratio R1/R2 is less than 1at any temperature in the range of 700° C. to 900° C. Preferably, thecross-sectional area ratio S2/S1 is 0.87 or less, more preferably 0.80or less. The cross-sectional area ratio S2/S1 may be controlled, forexample, by at least either the operations of controlling the width W1of the lead side curved portion 77 b to be larger than the width W2 ofthe straight portions 78, or the operation of controlling the thicknessD1 of the lead side curved portion 77 b to be larger than the thicknessD2 of the straight portions 78. For example, in the case of widthW1>width W2, any relationship of thickness D1<thickness D2, thicknessD1=thickness D2, and thickness D1>thickness D2 may hold true, as long asthe cross-sectional area ratio S2/S1 is less than 1. Similarly, in thecase of thickness D1>thickness D2, any relationship of width W1<widthW2, width W1=width W2, and width W1>width W2 may hold true, as long asthe cross-sectional area ratio S2/S1 is less than 1. Cross-sectionalareas S1 and S2 are defined by the averages thereof of the lead sidecurved portion 77 b and the straight portions 78, respectively, as withunit resistances R1 and R2. It is also preferable that the cross sectionof the lead side curved portion 77 b (taken in the directionperpendicular to the length direction thereof) at least at the closestpoint (at points P1 and P3) thereof to the lead section 79 have a largerarea than cross-sectional area S2. In the present embodiment, thestraight portions 78 have the same cross-sectional area(=cross-sectional area S2) at any position. The two non-lead side curvedportions 77 a have the same cross-sectional area as the straightportions 78. The cross-sectional area of the lead side curved portion 77b is the same as the cross-sectional area of the straight portions 78 atthe joint with the straight portions 78, and is gradually increased withincreasing distance from the straight portion 78. Hence, each of the atleast one lead side curved portion 77 b has a cross section varying soas to be largest at the center in the left-right direction(cross-section passing through the point most protruding to the rearside). The cross-sectional area ratio S2/S1 may be 0.5 or more. WidthsW1 and W2 may be in the range of 0.05 mm to 1.5 mm. Thicknesses D1 andD2 may be in the range of 0.003 mm to 0.1 mm.

The lead section 79 includes the first lead 79 a located to the rearleft of the heating section 76, and the second lead 79 b located to therear right of the heating section 76. The first and the second lead 79 aand 79 b are used for supplying electricity to the heating section 76and are connected to the heater connector electrode 71. The first lead79 a is a positive lead, and the second lead 79 b is a negative lead. Byapplying a voltage between the first and the second lead 79 a and 79 b,a current flows to the heating section 76, and thus the heating section76 heats. The lead section 79, which is an electric conductor, has alower resistance per unit length than the heating section 76. Unlike theheating section 76, the lead section 79 therefore hardly heats duringpower supply. For example, the lead section 79 may be made of a materialhaving a lower volume resistivity than the material of the heatingsection 76 or have a larger cross-sectional area than the heatingsection 76 so as to have a lower resistance per unit length. In thepresent embodiment, the volume resistivity of the lead section 79 isreduced by adding a noble metal with a higher content than that in theheating section 76, and the cross-sectional area of the lead section 79is made larger than that of the heating section 76 by controlling thewidth of the lead section 79 to be larger than that of the heatingsection 76. The width of the lead section 79 in the left-right directionis substantially the same as the forward straight portion 78 at thejoint therebetween and is increased toward the rear side.

A method for manufacturing the gas sensor 100 having the above-describedstructure will now be described. First, six ceramic green sheetscontaining an oxygen ion conductive solid electrolyte, such as zirconia,as a ceramic component, are prepared. In these green sheets, alignmentholes used for printing or stacking, through-holes, and the like havebeen formed in advance. The green sheet for the spacer layer 5 isprovided therein with a space for the gas delivering section that hasbeen formed by punching. Then, different patterns are formedrespectively on the ceramic green sheets for the first substrate layer1, the second substrate layer 2, the third substrate layer 3, the firstsolid electrolyte layer 4, the spacer layer 5, and the second solidelectrolyte layer 6 by pattern printing and drying. More specifically,the patterns to be formed are for, for example, the above-describedelectrodes, lead wires connected to the electrodes, the air introducinglayer 48, the heater 72, and the like. For the pattern printing, apattern forming paste prepared according to the characteristics requiredfor the intended pattern is applied onto a green sheet by a known screenprinting technique. The pattern forming paste for the heater 72 is amixture containing the raw material (for example, a noble metal andceramic particles) for the heater 72, an organic binder, and an organicsolvent.

The pattern for the heater 72 is formed so that the unit resistanceratio R1/R2 will be less than 1, that is, so that the cross-sectionalarea ratio S2/S1 will be less than 1. For example, for satisfying therelationship width W1>width W2, a mask enabling such a pattern to beformed is used. For satisfying the relationship thickness D1>thicknessD2, for example, the pattern forming paste for the lead side curvedportion 77 b is prepared so as to have a higher viscosity than thepattern forming paste for the straight portions 78, or the number ofprinting operations for forming the pattern for the lead side curvedportion 77 b is increased.

After the various patterns are formed, the green sheets are dried. Thedrying process is performed by using a known drying means. After thepattern printing and drying are finished, the green sheets are subjectedto printing of an adhesive paste for stacking and bonding the greensheets, followed by drying. Then, the resulting green sheets are stackedin a predetermined order with the holes in the sheets aligned. The stackis subjected to pressure bonding for forming a multilayer body by beingheated to a predetermined temperature under a predetermined pressure.The resulting multilayer body includes a plurality of sensor elements101. The multilayer body is cut into pieces having a size correspondingto the sensor element 101. The pieces of the multilayer body aresintered at a predetermined temperature to yield sensor elements 101.

The sensor element 101 thus produced is incorporated in a sensorassembly, and a protective cover, for example, is attached to theassembly. Thus, a gas sensor 100 is completed. The process formanufacturing a gas sensor has been known except for the technique ofsetting the unit resistance ratio R1/R2 to less than 1, and is disclosedin, for example, International Publication No. 2013/005491.

For use of the resulting gas sensor 100, the heater 72 is connected to apower source (for example, the alternator of an automobile) through theheater connector electrode 71, and a direct voltage (for example, 12 Vto 14 V) is applied between the first lead 79 a and the second lead 79b. The applied voltage causes a current to flow in the heating section76. The heating section 76 thus heats. Thus, the entirety of the sensorelement 101 is controlled to a temperature (for example, 700° C. to 900°C.) at which the solid electrolyte (of layers 1 to 6) is activated. Atthis time, in portions of the heater 72 (the heating section 76 and thelead section 79) having a higher potential gradient (=potentialdifference/distance between surfaces) between the surfaces, impurities(for example, oxides of alkali metal or alkaline-earth metal) in theheater 72 or the heater insulating layer 74 can be ionized. Ions thusproduced include cations such as sodium ion (Na⁺), calcium ion (Ca²⁺),and magnesium ion (Mg²⁺), and anions such as oxide ion (O²⁻). If theseions are produced, cations are attracted to the low potential side, andanions are attracted to the high potential side. Thus ions move in theheater insulating layer 74 in some cases. This phenomenon is calledmigration. Ions that have migrated to a portion of the heater 72 reactwith a constituent in the portion (for example, react with a noble metalin the heater 72), thus degrading the heater 72 in such a manner that aline of the pattern is thinned or broken. In addition, ionization ofimpurities as described above is more likely to occur as temperatureincreases. In the present embodiment, the lead side curved portion 77 bis close to the lead section 79, and accordingly, the potential gradientbetween the lead side curve portion 77 b and the lead section 79 ishigher than that between the other portions of the heating section 76and the lead section 79. More specifically, the region between points P1and P2 has a short distance L1 and has a potential difference about halfthat of the region between the first and the second lead 79 a and 79 b(for example, 12 V to 14 V), accordingly having a relatively highpotential gradient. Similarly, the region between points P3 and P4 has arelatively high potential gradient. Accordingly, migration is likely tooccur between the lead side curved portion 77 b and the first lead 79 a.Thus, the lead side curved portion 77 b is likely to be degraded bymigration of cations to point P1, and the first lead 79 a is likely tobe degraded by migration of anions to point P2. Migration is also likelyto occur between the lead side curved portion 77 b and the second lead79 b. Thus, the second lead 79 b is likely to be degraded by migrationof cations to point P4, and the lead side curved portion 77 b is likelyto be degraded by migration of anions to point P3. In the presentembodiment, however, the unit resistance ratio R1/R2 is set to be lessthan 1 at one or more temperatures in the range of 700° C. to 900° C.Thus, the lead side curved portion 77 b has a lower heating density thanthe straight portions 78 at one or more temperatures in the range of700° C. to 900° C., and accordingly, the temperature increase of thelead side curved portion 77 b is suppressed. Since the temperatureincrease of the lead side curved portion 77 b, in which migration isless likely to occur, and of the surroundings thereof is suppressed,ionization as described above is prevented, and migration is prevented.Thus, the lead side curved portion 77 b and the first and the secondlead 79 a and 79 b can be prevented from being degraded by migration.Thus, the degradation of the heater 72 is reduced, so that the lifetimeof the heater 72 is extended.

The relationship between the components of the present embodiment andthe components of the present invention will now be clearly described.The heater section 70 of the present embodiment corresponds to theceramic heater of the present invention; a stack of the first substratelayer 1, the second substrate layer 2, and the third substrate layer 3corresponds to the plate-like ceramic body; the heater 72 corresponds tothe heating element; the straight portion 78 corresponds to the straightportion; and the lead side curved portion 77 b corresponds to the leadside curved portion. The closest lead side curved portion 77 b to thelead section corresponds to the closest to the lead section of the atleast one lead side curved portion of the first ceramic heater of thepresent invention, and the closest lead side curved portion 77 b to thepositive lead corresponds to the closest to the positive lead of the atleast one lead side curved portion of the second ceramic heater.

According to the gas sensor 100 of the present embodiment, the heatersection 70 includes a plate-like ceramic body (the first substrate layer1, the second substrate layer 2, and the third substrate layer 3) havinga longitudinal direction and a short-length direction, and a heater 72disposed within the plate-like ceramic body and including a lead section79 and a heating section 76 connected to the lead section 79. Theheating section 76 includes a straight portion 78 extending along thelongitudinal direction, and a lead side curved portion 77 b connected toone of the ends of the straight portion 78 closer to the lead section79. The lead side curved portion 77 b that is the closest lead sidecurved portion to the lead section and to the positive lead has a lowerresistance per unit length than the straight portion 78 at one or moretemperatures in the range of 700° C. to 900° C. Accordingly, the leadside curved portion 77 b has a lower heating density than the straightportion 78 at one or more temperatures in the range of 700° C. to 900°C., and thus the temperature increase of the lead side curved portion 77b is suppressed. Thus, at least either the lead side curved portion 77 bor the lead section 79 can be prevented from being degraded bymigration, and the degradation of the heater 72 is reduced. Apart fromthe degradation resulting from migration, the heater 72 is more likelyto be degraded by oxidation (for example, oxidation of Pt that is anoble metal in the heating section 76) with increasing temperature.Since the temperature increase of the lead side curved portion 77 b issuppressed, the lead side curved portion 77 b is prevented from beingdegraded by oxidation, as well from degraded by migration.

In addition, by setting the unit resistance ratio R1/R2 to be 0.87 orless at one or more temperatures in the above temperature range, thedegradation of the heater 72 resulting from migration is reduced moreeffectively. Furthermore, by setting the unit resistance ratio R1/R2 tobe 0.80 or less at one or more temperatures in the above temperaturerange, the degradation of the heater 72 resulting from migration isreduced still more effectively. Since the cross-section of the lead sidecurved portion 77 b taken in the direction perpendicular to the lengthdirection thereof has a larger area than that of the straight portions78, the resistance per unit length (unit resistance R1) of the lead sidecurved portion 77 b tends to be lower than the unit resistance R2 of thestraight portions 78. Also, by setting the cross-sectional area of thelead side curved portion 77 b to be larger than that of the straightportions 78, the amount per unit length of noble metal in the lead sidecurved portion 77 b is larger than that in the straight portions 78.Even if degradation resulting from migration occurs in the lead sidecurved portion 77 b, therefore, the lead side curved portion 77 b willtake a long time to be entirely degraded. Thus, adverse effects of suchdegradation is unlikely to be produced and, for example, disconnectionresulting from degradation is unlikely to occur. Thus, the lifetime ofthe lead side curved portion 77 b is extended. When the cross-sectionalarea ratio S2/S1 is 0.87 or less, the unit resistance ratio R1/R2 tendsto be 0.87 or less at one or more temperatures in the above temperaturerange. Also, when the cross-sectional area ratio S2/S1 is 0.80 or less,the unit resistance ratio R1/R2 tends to be 0.80 or less at one or moretemperatures in the above temperature range.

Furthermore, the heating section 76 may have four or more straightportions 78 arranged along the short-length direction (left-rightdirection) of the plate-like ceramic body. In this structure, the atleast one lead side curved portion 77 b connects a pair of the straightportions 78 adjacent to each other in the short-length direction at oneof the ends of each adjacent straight portion closer to the lead section79, and the heating section 76 has a plurality of non-lead side curvedportions 77 a, each connecting a pair of the straight portions 78adjacent to each other in the short-length direction at one of the endsof each adjacent straight portion far from the lead section 79. Thesensor element 101 includes the heater section 70 and detects theconcentration of a specific gas in a measurement-object gas. The gassensor 100 includes the sensor element 101.

The present invention is not limited to the above-described embodiment,and it should be appreciated that various embodiments can be applied tothe invention within the technical scope of the invention.

For example, while the cross-sectional area ratio S2/S1 is 1 or less inthe above-described embodiment, it is not limited thereto as long as theunit resistance ratio R1/R2 is less than 1 at one or more temperaturesin the range of 700° C. to 900° C. For example, the lead side curvedportion 77 b may have a lower volume resistivity than the straightportions 78 at one or more temperatures in that temperature range.Hence, the volume resistivity ratio ρ1/ρ2 of the volume resistivity ρ1[cm] of the lead side curved portion 77 b to the volume resistivity ρ2[μΩ·cm] of the straight portions 78 may be less than 1 at one or moretemperatures in the above temperature range. The unit resistance ratioR1/R2 thus can be set to be less than 1 at one or more temperatures inthe above temperature range. Consequently, the degradation of the heater72 resulting from migration can be reduced. Preferably, the volumeresistivity ratio ρ1/ρ2 is 0.87 or less, more preferably 0.80 or less,at one or more temperatures in the above temperature range. For example,by setting the noble metal (electroconductive material) content in thelead side curved portion 77 b to be higher than that in the straightportions 78, volume resistivity ρ1 can be set to be lower than volumeresistivity ρ2. Furthermore, by setting the noble metal content in thelead side curved portion 77 b to be higher than that in the straightportions 78, the amount per unit length of noble metal in the lead sidecurved portion 77 b can be set to be higher than that in the straightportions 78, as in the case of relatively increasing the cross-sectionalarea of the lead side curved portion 77 b. When the amount per unitlength of noble metal in the lead side curved portion 77 b is relativelyincreased, adverse effects of degradation is unlikely to be produced.Thus, the lifetime of the lead side curved portion 77 b is extended.Alternatively, by adding a noble metal (rhodium, gold, or the like)having a lower volume resistivity than platinum to the lead side curvedportion 77 b in addition to or instead of adding platinum while thestraight portions 78 mainly contains platinum, volume resistivity ρ1 canbe set to be lower than volume resistivity ρ2. In other words, the leadside curved portion 77 b may contain a noble metal that is not containedin the straight portions 78 and has a lower volume resistivity than thenoble metal contained in the straight portions 78. Alternatively, byadding to the lead side curved portion 77 b a material whose resistancehas a lower temperature coefficient (unit: [%/° C.]) than the noblemetal mainly contained in the lead side curved portion 77 b, with ahigher proportion than in the straight portions 78, volume resistivityρ1 can be set to be lower than volume resistivity ρ2 at one or moretemperatures in the above temperature range. Examples of the materialwhose resistance has a low temperature coefficient include Nichrome(alloy containing nickel (Ni) and chromium (Cr)), Kanthal (registeredtrademark, alloy containing iron, chromium, and aluminum), andmolybdenum disilicide (MoSi₂). Volume resistivities ρ1 and ρ2 aredefined as the averages thereof in the lead side curved portion 77 b andthe straight portions 78, respectively, as with unit resistances R1 andR2. The volume resistivity ratio ρ1/ρ2 may be 0.5 or more at anytemperature in the above temperature range.

It may be combined in the heater section 70 to set the cross-sectionalarea ratio S2/S1 to be less than 1 and to set the volume resistivityratio ρ1/ρ2 to be less than 1. For example, the product (=unitresistance ratio R1/R2) of the cross-sectional area ratio S2/S1 and thevolume resistivity ratio ρ1/ρ2 may be set to be less than 1, 0.87 orless, or 0.80 or less at one or more temperatures in the abovetemperature range. When the cross-sectional area ratio S2/S1 is lessthan 1, the lead side curved portion 77 b and the straight portions 78may be made of different materials.

The shape (pattern) of the heater 72 in the heater section 70 is notlimited to the above-described embodiment. The heating section 76 of theheater 72 includes the lead side-curved portion 77 b and the straightportions 78 and is otherwise not limited. The straight portions 78 arenot necessarily parallel to each other as long as they extend along thelongitudinal direction (front-rear direction) of the heater section 70.FIG. 3 is an illustrative representation of a heater 72A according to amodification. In this heater 72A, two of the four straight portions 78,located at the center in the left-right direction extend along thelongitudinal direction, but are inclined with respect to thelongitudinal direction. More specifically, the second straight portion78 from the left is inclined to the left while extending to the rearside, and the second straight portion 78 from the right is inclined tothe right while extending to the rear side. This shape enables thecurved portions 77 to have a larger radius (radius of curvature) thanthat of the heater 72 shown in FIG. 2 . In other words, the width of theheating section 76 in the left-right direction can be reduced withoutreducing the radius of curvature of the lead side curved portion 77 b.Also, unlike the shape shown in FIG. 2 , the joint of the lead section79 with the forward straight portion 78 has a wider width than thestraight portion 78. This lead section 79 may have the same shape as inFIG. 2 , or the lead section 79 of the heater 72 shown in FIG. 2 mayhave the same shape as in FIG. 3 . The heater 72A of this modificationcan produce the same effect as in the above-described embodiment. Forexample, by setting the unit resistance ratio R1/R2 to be less than 1 atone or more temperatures in the range of 700° C. to 900° C., thedegradation of the heater 72 resulting from migration can be reduced.

While the above-described embodiment has been illustrated for theheating section 76 including three curved portions 77 and four straightportions 78, the heating section is not limited thereto as long as theheating section 77 includes at least one straight portion 78 and atleast one lead side curved portion 77 b connected to the straightportion 78. For example, the number of the curved portions 77 may bethree or more, or one or two, and the number of the straight portions 78may be four or more, or three or less. The number of the straightportions 78 may be an even number of four or more. While theabove-described embodiment has been illustrated for the case of havingtwo non-lead side curved portions 77 a and one lead side curved portion77 b, the numbers of these curved portions may be varied according tothe number of straight portions 78. For example, the number of thenon-lead side curved portions 77 a may be one or two or more, as long asthe number of the lead-side curved portions 77 b is one or more.

FIG. 4 is an illustrative representation of a heater 72B according to amodification in which the heating section 76 includes two lead sidecurved portions 77 b. The heating section 76 of the heater 72B includessix straight portions 78 arranged in the short-length direction, threenon-lead side curved portions 77 a, and two lead side curved portions 77b (lead side curved portions 77 b 1 and 77 b 2). If distance L1 is equalto distance L2 in this heater 72B, both of the lead side curved portions77 b 1 and 77 b 2 are the closest to the lead section of the lead sidecurved portions. In the case of distance L1<distance L2, the lead sidecurved portion 77 b 1 is the closest lead side curved portion; in thecase of distance L1>distance L2, the lead side curved portion 77 b 2 isthe closest lead side curved portion. The closest to the positive leadof the lead side curved portions is the lead side curved portion 77 b 1irrespective of the lengths of distances L1 and L2. Thus, when thenumber of the lead side curved portions 77 b is two or more, the closestlead side curved portion to the lead section and the closest lead-sidecurved portion to the positive lead may be different. In this case, atleast either the closest lead side curved portion to the lead section orthe closest lead side curved portion to the positive lead satisfies theunit resistance ratio R1/R2 of less than 1 at one or more temperaturesin the above temperature range. Thus, the degradation of the heater 72resulting from migration can be reduced as in the case of theabove-described embodiment. More specifically, at least either theclosest lead side curved portion to the lead section 79 or the leadsection 79 can be prevented from being degraded as long as the unitresistance R1 of the closest lead side curved portion is lower than unitresistance R2. Also, at least either the closest lead side curvedportion to the positive lead or the first lead 79 a (positive lead) canbe prevented from being degraded as long as the unit resistance R1 ofthe closest lead side curved portion to the positive lead is lower thanunit resistance R2. In comparison between the lead section 79 and thelead side curved portion 77 b, the lead side curved portion 77 b, whichis likely to have higher temperature, is more likely to be degraded thanthe lead section 79. If the cross section of the lead section 79 takenin the direction perpendicular to length direction thereof has a largerarea than that of the lead side curved portion 77 b, the lead section 79will take a long time to be entirely degraded, and accordingly, the leadside curved portion 77 b is more likely to be degraded. If the number ofthe lead side curved portions 77 b is two or more, the unit resistanceR1 that is the resistance per unit length of at least either the closestlead side curved portion to the positive lead or the closest to thenegative lead of the two or more lead side curved portions 77 bsatisfies the unit resistance ratio R1/R2 of less than 1 at one or moretemperatures in the above temperature range. Also, the unit resistanceR1 that is the resistance per unit length of each of the closest leadside curved portion to the positive lead and the closest lead sidecurved portion to the negative lead may satisfy the unit resistanceratio R1/R2 of less than 1 at one or more temperatures in the abovetemperature range.

In the above-described embodiment, the lead section 79 includes thefirst and the second lead 79 a and 79 b for supplying electricity. Inaddition, the lead section 79 may further include any other lead, suchas a lead used for measuring voltage. FIG. 5 is an illustrativerepresentation of a heater 72C according to a modification in this case.In the heater 72C, the lead section 79 includes a third and a fourthlead 79 c and 79 d used for measuring voltage, in addition to the firstand the second lead 79 a and 79 b for supplying electricity. The thirdlead 79 c is connected to the joint between the first lead 79 a and thecorresponding straight portion 78 so as to be connected to the firstread 79 a in parallel. The fourth lead 79 d is connected to the jointbetween the second lead 79 b and the corresponding straight portion 78so as to be connected to the second read 79 b in parallel. In thisheater 72C, the voltage between the third and the fourth lead 79 c and79 d can be measured in a state where a voltage is applied between thefirst and the second lead 79 a and 79 b. Thus, the voltage between bothends of the heating section 76 can be accurately measured, eliminatingan error resulting from the resistances of the first and the second lead79 a and 79 b (what is called four-terminal method). In the heater 72C,the first and the third lead 79 a and 79 c correspond to the positivelead, and the second and the fourth lead 79 b and 79 d corresponds tothe negative lead. In this heater 72C, the lead side curved portion 77 bis close to the third and the fourth lead 79 c and 79 d. Accordingly,the potential gradient between the lead side curved portion 77 b and thethird lead 79 c (for example, between points P1 and P2) and between thelead side curved portion 77 b and the fourth lead 79 d (for example,between points P3 and P4) tends to increase. Therefore, by setting theunit resistance R1 of the lead side curved portion 77 b that is theclosest to the lead section and to the positive lead of the at least onelead side curved portion to be lower than unit resistance R2, thedegradation of the heater 72 resulting from migration can be reduced asin the case of the above-described embodiment. In the heater 72C shownin FIG. 5 , the lead section 79 does not necessarily include the fourthlead 79 d. In this instance, the voltage between the first and thesecond lead 79 a and 79 c (=voltage drop of the first lead 79 a) issubstantially equal to the voltage drop of the second lead 79 b.Therefore, the voltage between both ends of the heating section 76 canbe accurately obtained as the remainder of the subtraction of thevoltage between the first and the third lead 79 a and 79 c from thevoltage between the third and the second lead 79 c and 79 b. In the caseof not including the fourth lead 79 d, as well as in the above case, bysetting the unit resistance R1 of the lead side curved portion 77 b thatis the closest to the lead section and to the positive lead of the atleast one lead side curved portion to be lower than unit resistance R2,the degradation of the heater 72 resulting from migration can be reducedas in the case of the above-described embodiment.

While the above-described embodiment has been illustrated for the casein which the cross-sectional area of the lead side curved portion 77 bincreases as the distance from the straight portion 78 increases, it isnot limited thereto. For example, the cross-sectional area of the leadside curved portion 77 b may be the same (=cross-sectional area S1) atany point. In this instance, a step may be formed at the joint betweenthe straight portion 78 and the lead side curved portion 77 b. It is,however, preferable that there be no step in the heating section 76. Forgiving different cross-sectional areas to the lead side curved portion77 b and the straight portion 78, it is preferable to gradually vary thecross-sectional area of the lead side curved portion 77 b, as shown inFIG. 2 . Also, the straight portions 78 have the same cross-sectionalarea (=cross-sectional area S2) at any position. It is however notlimited thereto. For example, the straight portions 78 may have aportion whose cross-sectional area gradually varies. If the heatingsection 76 includes a plurality of lead side curved portions 77 b, atleast one of the plurality of lead side curved portions 77 b may have adifferent cross-sectional area from the other lead side curved portions77 b. The same applies to the case of a plurality of straight portions78.

While the above-described embodiment has been illustrated for the casein which the two non-lead side curved portions 77 a has the samecross-sectional area as the cross-sectional area of the straightportions 78, it is not limited thereto. The lead side curve portion 77 band the straight portions 78 satisfy the unit resistance ratio R1/R2 ofless than 1 at one or more temperatures in the above temperature rangeand are otherwise not limited. For example, the cross-sectional area ofthe non-lead side curved portions 77 a may be smaller than or largerthan that of the straight portions 78. Similarly, the cross-sectionalarea of the non-lead side curved portions 77 a may be smaller than,larger than, or the same as that of the lead side curve portion 77 b.

While the above-describe embodiment has been illustrated for the heater72 in the form of a strip, it is not limited thereto and may be in theform of a line (for example, having a circular or oval cross section).

While the above-described embodiment has been illustrated for the gassensor 100 including the heater section 70, the present invention may beembodied as an independent sensor element 101, or an independent heatersection 70, that is, an independent ceramic heater. While the heatersection 70 described above includes the first substrate layer 1, thesecond substrate layer 2, and the third substrate layer 3, it is notlimited thereto as long as it includes a plate-like ceramic bodysurrounding the heater 72. For example, the layer underlying the heater72 may be defined by only a single layer, but not by the two layers ofthe first substrate layer 1 and the second substrate layer 2. While theheater section 70 described above includes the heater insulating layer74, the heater insulating layer 74 may be omitted as long as theplate-like ceramic body (for example, the first substrate layer 1, thesecond substrate layer 2) surrounding the heater 72 is made of aninsulating material (such as alumina ceramic). The sensor element 101may have a length in the range of 25 mm to 100 mm in the front-reardirection, a width in the range of 2 mm to 10 mm in the left-rightdirection, and a thickness in the range of 0.5 mm to 5 mm in thevertical direction.

EXAMPLES

Examples in which sensor elements were specifically manufactured willnow be described. Experimental Examples 2 to 9 and 11 to 18 correspondto Examples of the present invention, and Experiment Examples 1 and 10correspond to Comparative Examples. It should be noted that theinvention is not limited to the examples below.

Experimental Examples 1 to 9

Sensor elements 101 as shown in FIGS. 1 and 2 were prepared asExperimental Examples 1 to 9 in accordance with the method formanufacturing the gas sensor 100 of the above-described embodiment.Experimental Examples 1 to 9 had the same structure except that thecross-sectional area ratio S2/S1 was varied as shown in Table 1 byvarying the width W1 of the lead side curved portion 77 b. The sensorelement 101 measured 67.5 mm in length in the front-rear direction, 4.25mm in width in the left-right direction, and 1.45 mm in thickness in thevertical direction. In Experimental Example 1, the width W1 of the leadside curved portion 77 b and the width W2 of the straight portions 78were each 0.25 mm. Also, in Experimental Example 1, the thickness D1 ofthe lead side curved portion 77 b and the thickness D2 of the straightportions 78 were each 0.01 mm. For producing the sensor element 101,ceramic green sheets were formed of a mixture of zirconia particlescontaining 4% by mole of yttria as a stabilizing agent, an organicbinder, and an organic solvent by tape formation. The electroconductivepaste for forming heating section 76 of the heater section 70 wasprepared as below. A preliminary mixture was prepared by preliminarilymixing 4% by mass of alumina particles, 96% by mass of Pt, and apredetermined amount of acetone as a solvent. An organic binder liquidprepared by dissolving 20% by mass of polyvinyl butyral in 80% by massof butyl carbitol was added to the preliminary mixture, and then butylcarbitol was added as needed to adjust the viscosity. Thus, theelectroconductive paste was prepared. In Experimental Example 1, thecurved portions 77 and the straight portions 78 were formed of the sameelectroconductive paste; hence the volume resistivity ratio ρ1/ρ2 was 1at any temperature in the range of 700° C. to 900° C. The same appliesto Experimental Examples 2 to 9. The electroconductive paste for thelead section 79 was prepared in the same manner as the electroconductivepaste for the heating section 76 except that the alumina content and thePt content were 2% by mass and 98% by mass, respectively.

Experimental Examples 10 to 18

Sensor elements 101 of Experimental Examples 10 to 18 were produced inthe same as in Experimental Example 1 except that the volume resistivityratio ρ1/ρ2 was varied as shown in Table 1. The volume resistivity ratioρ1/ρ2 was varied by varying the Pt content in the lead side curvedportion 77 b. In any of Experimental Examples 10 to 18, widths W1 and W2and thicknesses D1 and D2 were the same as in Experimental Example 1,and the cross-sectional area ratio S2/S1 was 1.00. In ExperimentalExamples 10 and 1, the cross-sectional area ratio S2/S1 and the volumeresistivity ratio ρ1/ρ2 were the same.

The volume resistivity ρ1 in Experimental Examples 10 to 18 was measuredusing test pieces prepared as below. An insulating paste for the heaterinsulating layer 74 was applied by printing onto a ceramic green sheetthat would be sintered into the second substrate layer 2.

Subsequently, an electroconductive paste prepared under the sameconditions as the respective electroconductive paste of the lead sidecurved portion 77 b of Experimental Examples 10 to 18 was applied ontothe insulating paste so as to form a rectangular parallelepiped. Then,the pastes were sintered under the same conditions as in ExperimentalExamples 10 to 18. The heating sections thus formed in the shape of arectangular parallelepiped were used as test pieces of ExperimentalExamples 10 to 18. Leads for measuring resistance were connected to eachrectangular parallelepipedal heating section. The resulting test piecewas heated to 700° C. to 900° C. in an electric furnace, and theresistance of the heating section was measured in this state. Thus,volume resistivity ρ1 was calculated using the length of the rectangularparallelepipedal heating section, the cross-sectional area, and themeasured resistance. Similarly, volume resistivity ρ2 was calculatedfrom the results of measurement using the test piece. In ExperimentalExamples 10 to 18, the volume resistivity ratio ρ1/ρ2 hardly varied inthe range of 700° C. to 900° C.

The durability (lifetime) of the heating section 76 of ExperimentalExamples 1 to 18 was examined. More specifically, a voltage was appliedto the lead section 79 for supplying electricity to the heater 72 sothat the heating section 76 would have a predetermined averagetemperature. Then, it was examined whether or not disconnection occurredin the heating section 76 within 2000 hours. When disconnection did notoccur beyond 2000 hours, the durability was rated as “A (excellent,better than practical level)”; when disconnection occurred in the rangeof more than 1000 to 2000 or less, the durability was rated as “B (good,practical level)”; and when disconnection occurred within 1000 hours,the durability was rated as “C (failure, worse than practical level)”.The durability of the heater 72 was examined in each of the cases inwhich the heating section 76 had an average temperature of 700° C., 750°C., 800° C., 850° C., and 900° C. The temperature of the heating section76 was controlled by varying the voltage to be applied to the leadsection 79. The temperature of the heating section 76 was indirectlymeasured by measuring the bottom temperature of the sensor element 101with a radiation thermometer. The results of the examinations are shownin Table 1. Table 1 also shows the unit resistance ratio R1/R2, thecross-sectional area ratio S2/S1, and the volume resistivity ratio ρ1/ρ2of each Experimental Example. The unit resistance ratio R1/R2 wascalculated as the product of the cross-sectional area ratio S2/S1 andthe volume resistivity ratio ρ1/ρ2.

TABLE 1 Average Temperature of Unit Resistance Cross-Sectional VolumeResistivity Heating Section (° C.) Ratio R1/R2 Area Ratio S2/S1 Ratio ρ1/ρ 2 700 750 800 850 900 Experimental Example 1 1.00 1.00 1.00 C C C CC Experimental Example 2 0.95 0.95 1.00 C C C C C Experimental Example 30.91 0.91 1.00 C C C C C Experimental Example 4 0.87 0.87 1.00 A B B B BExperimental Example 5 0.83 0.83 1.00 A A B B B Experimental Example 60.80 0.80 1.00 A A A A A Experimental Example 7 0.77 0.77 1.00 A A A A AExperimental Example 8 0.74 0.74 1.00 A A A A A Experimental Example 90.71 0.71 1.00 A A A A A Experimental Example 10 1.00 1.00 1.00 C C C CC Experimental Example 11 0.95 1.00 0.95 C C C C C Experimental Example12 0.91 1.00 0.91 C C C C C Experimental Example 13 0.87 1.00 0.87 A B BB B Experimental Example 14 0.83 1.00 0.83 A A B B B ExperimentalExample 15 0.80 1.00 0.80 A A A A A Experimental Example 16 0.77 1.000.77 A A A A A Experimental Example 17 0.74 1.00 0.74 A A A A AExperimental Example 18 0.71 1.00 0.71 A A A A A A: Excellent, B: Good,C: Failure

Table 1 shows that disconnection in the heater 72 is less likely tooccur as the unit resistance R1/R2 decreases. It was also observed thatdisconnection in the heater 72 less likely to occur even at hightemperatures as the unit resistance R1/R2 decreases. In ExperimentalExamples 4 to 9 and 13 to 18 in which the unit resistance ratio R1/R2was set to 0.87 or less, the examination results were rated as A(excellent) or B (good) at any temperature in the range of 700° C. to900° C. In Experimental Examples 6 to 9 and 15 to 18 in which the unitresistance ratio R1/R2 was set to 0.80 or less, the examination resultswere rated as A (excellent) at any temperature in the range of 700° C.to 900° C. In any of the Experimental Examples rated as B (good) or C(failure), disconnection occurred in the lead side curved portion 77 b.The comparison between Experimental Examples 1 to 9 and ExperimentalExamples 10 to 18 shows that the examples in which the unit resistanceratio R1/R2 was the same as each other produced the same results betweenthe case of varying the cross-sectional area ratio S2/S1 and the case ofvarying the volume resistivity ratio ρ1/ρ2. When the cross-sectionalarea ratio S2/S1 was varied by varying the thickness D1 of the lead sidecurved portion 77 b, the same results as in Experimental Examples 1 to 9were produced.

The present application claims priority from Japanese Patent ApplicationNo. 2015-164211 filed on Aug. 21, 2015, the entire contents of which areincorporated herein by reference.

What is claimed is:
 1. A ceramic heater comprising: a plate-like ceramic body having a longitudinal direction and a short-length direction; and a heating element disposed within the plate-like ceramic body, the heating element including a lead section and a heating section connected to the lead section, wherein the heating section includes a straight portion whose length direction is along the longitudinal direction and at least one lead side curved portion connected to one of the ends of the straight portion closer to the lead section, and wherein the closest to the lead section of the at least one lead side curved portion has a lower resistance per unit length than the straight portion at one or more temperatures in the range of 700° C. to 900° C., wherein the heating section is formed of a material comprising a noble metal, and wherein the straight portion has a same cross sectional area at any position.
 2. The ceramic heater according to claim 1, wherein the ratio R1/R2 of unit resistance R1 [μΩ/mm] being the resistance per unit length of the closest of the at least one lead side curved portion to unit resistance R2 [μΩ/mm] being the resistance per unit length of the straight portion is 0.87 or less at one or more temperatures in the temperature range.
 3. The ceramic heater according to claim 2, wherein the unit resistance ratio R1/R2 is 0.80 or less at one or more temperatures in the temperature range.
 4. The ceramic heater according to claim 1, wherein a cross section of the closest of the at least one lead side curved portion, taken in a direction perpendicular to a length direction thereof has a larger area than a cross section of the straight portion, taken in a direction perpendicular to a length direction thereof.
 5. The ceramic heater according to claim 4, wherein the ratio S2/S1 of the area S2 [mm²] of the cross section of the straight portion taken in the direction perpendicular to the length direction thereof to the area S1 [mm²] of the cross section of the closest of the at least one lead side curved portion taken in the direction perpendicular to the length direction thereof is 0.87 or less.
 6. The ceramic heater according to claim 5, wherein the cross sectional area ratio S2/S1 is 0.80 or less.
 7. The ceramic heater according to claim 1, wherein the closest of the at least one lead side curved portion has a lower volume resistivity than the straight portion at one or more temperatures in the temperature range.
 8. The ceramic heater according to claim 7, wherein the ratio μ1/ρ2 of the volume resistivity ρ1 [μΩ·cm] of the closest of the at least one lead side curved portion to the volume resistivity ρ2 [μΩ·cm] of the straight portion is 0.87 or less at one or more temperatures in the temperature range.
 9. The ceramic heater according to claim 8, wherein the volume resistivity ratio μ1/ρ2 is 0.80 or less at one or more temperatures in the temperature range.
 10. The ceramic heater according to claim 1, wherein the heating section includes four or more straight portions arranged along the short-length direction, and the at least one lead side curved portion connects a pair of the straight portions adjacent to each other in the short-length direction at one of the ends of each adjacent straight portion closer to the lead section, and wherein the heating section includes a plurality of non-lead side curved portions, each connecting a pair of the straight portions adjacent to each other in the short-length direction at one of the ends of each adjacent straight portion far from the lead section.
 11. A sensor element adapted to detect the concentration of a specific gas in a measurement-object gas, the sensor element comprising the ceramic heater according to claim
 1. 12. A gas sensor comprising the sensor element according to claim
 11. 13. A ceramic heater comprising: a plate-like ceramic body having a longitudinal direction and a short-length direction; and a heating element disposed within the plate-like ceramic body, the heating element including a lead section having a positive lead and a heating section connected to the lead section, wherein the heating section includes a straight portion whose length direction is along the longitudinal direction and at least one lead side curved portion connected to one of the ends of the straight portion closer to the lead section, and wherein the closest to the positive lead of the at least one lead side curved portion has a lower resistance per unit length than the straight portion at one or more temperatures in a range of 700° C. to 900° C., wherein the heating section is formed of a material comprising a noble metal, and wherein the straight portion has a same cross sectional area at any position.
 14. The ceramic heater according to claim 13, wherein the ratio R1/R2 of unit resistance R1 [μΩ/mm] being the resistance per unit length of the closest of the at least one lead side curved portion to unit resistance R2 [μΩ/mm] being the resistance per unit length of the straight portion is 0.87 or less at one or more temperatures in the temperature range.
 15. The ceramic heater according to claim 14, wherein the unit resistance ratio R1/R2 is 0.80 or less at one or more temperatures in the temperature range.
 16. The ceramic heater according to claim 13, wherein a cross section of the closest of the at least one lead side curved portion, taken in a direction perpendicular to a length direction thereof has a larger area than a cross section of the straight portion, taken in a direction perpendicular to a length direction thereof.
 17. The ceramic heater according to claim 13, wherein the closest of the at least one lead side curved portion has a lower volume resistivity than the straight portion at one or more temperatures in the temperature range.
 18. The ceramic heater according to claim 13, wherein the heating section includes four or more straight portions arranged along the short-length direction, and the at least one lead side curved portion connects a pair of the straight portions adjacent to each other in the short-length direction at one of the ends of each adjacent straight portion closer to the lead section, and wherein the heating section includes a plurality of non-lead side curved portions, each connecting a pair of the straight portions adjacent to each other in the short-length direction at one of the ends of each adjacent straight portion far from the lead section.
 19. A sensor element adapted to detect the concentration of a specific gas in a measurement-object gas, the sensor element comprising the ceramic heater according to claim
 13. 20. A gas sensor comprising the sensor element according to claim
 19. 